Unlike live vaccines that are quantified by in vitro titration, the potency of inactivated or subunit vaccines normally requires an in vivo test for each batch prior to its release for public use [1], although a number of exceptions exist e.g. the SRID (single radial immunodiffusion) potency test for the influenza vaccine and the use of ELISA for hepatitis B vaccines.
Typical in vivo tests involve an immunisation-challenge test using small rodents (mice or rats) as the experimental model. Depending on the type of vaccine, different endpoints are used, such as death/survival ratios (whole cell pertussis, diphtheria toxoid and tetanus toxoid, rabies vaccine), clinical signs (diphtheria, tetanus) or colonisation (whole cell and acellular pertussis). By establishing a dose-response curve in parallel to a standard preparation with known potency, the potency of the vaccine can be expressed relative to that preparation e.g. in standard units.
A challenge model is not always available. In those cases potency testing is usually limited to serological responses, with antibody responses being measured after immunisation of test animals. At least part of the functionality of these antibodies can be determined by their ability to neutralise the pathogen in vitro or to their ability to kill bacteria in the presence of complement (such as the serum bactericidal antibody assay, or SBA, for meningococcus).
The SBA assay is useful but cumbersome, and involves the sacrifice of many mice. As explained in reference 1 it is thus desirable to provide in vitro alternatives for assessing vaccine potency.
One in vitro assay for analysing MenB vaccines is the “MATS” ELISA test disclosed in references 2 and 3. The relative potency measured by MATS was shown to correlate with the ability of MenB strains to be killed in SBA.
The MATS test is used to evaluate the strain coverage of a MenB vaccine, rather than to analyse the vaccine's immunogenicity. There remains a need for further and improved in vitro assays for assessing the immunogenicity of meningococcal vaccines. Such in vitro assays could be used to confirm that a particular vaccine will have an expected in vivo activity in human recipients.